Decade or the like and components thereof

ABSTRACT

A decade counter and/or printer wherein a clapper is pivotally connected by means of a cylindrical enlargement to a pawl carrying arm. This arrangement has an unusually long useful life. The pawl has an adjustable stop for character alignment. Two identical contact brush assemblies may be located in at least two different places. This makes a cost saving possible.

United States Patent Einem et al.

[ Oct. 21, 1975 [54] DECADE OR THE LIKE AND 3,238,359 3/1966 Einem235/92 C COMPONENTS THEREOF 3,268,714 8/l966 Wales 235/92 C [75]Inventors: Robert E. Einern, Northridge;

Raymond vertefeui?! Lakeview Primary Examiner-Joseph M. Thesz, Jr.Terrace both of Attorney, Agent, or FirmA. Donald Stolzy [73] Assignee:International Telephone and Telegraph Corporation, New York, [57]ABSTRACT [22] Filed: July 17, 1972 [21] APPL NO; 272,421 A decadecounter and/or printer wherein a clapper is pivotally connected by meansof a cylindrical enlargement to a pawl carrying arm. This arrangementhas an 235/92 235/92 235/92 R unusually long useful life. The pawl hasan adjustable G061" 006M 1/276 stop for character alignment. Twoidentical contact Fleld Of Search C, brush assemblies may be 10cated inat least two different places. This makes a cost saving possible. [56]References Cited UNITED STATES PATENTS 23 Claims, 19 Drawing Figures2,323,235 6/1943 Nordenswan 235/92 C o I L Sheet 1 of 5 3,914,583

U.S Patent Oct. 21, 1975 U.S. Patent Oct.21, 1975 Sheet 2 of5 3,914,583

Sheet 3 of 5 3,914,583

Patent Oct. 21, 1975 US. Patent Oct. 21, 1975 Sheet4of5 3,914,583

Sheet 5 of 5 3,914,583

U.S. Patent Oct. 21, 1975 accurate indication of'a number by analignment of decade number wheels. See U.S. Pat. Nos. 3,429,258;3,566,196; and 3,238,359. I-Iowevensuch an arrangement has beenexpensiveto manufacture. Further, the clapper and pawl arm bearingconnection of prior art decades hasconsistently had ashort .useful life.

I SUMMARY OF THE INVENTION 1 In accordancewith the device. of thepresentinvention, the abovedescribed and other disadvantages of theprior art are overcome by providing an adjustable stop for a decade pawlto make possible a low cost alignment of characters or all the digits ofanumber indicated by a counter. I 1

Another feature of the inventionresides in'the use of a low cost andlonglife clapper and pawl arm bearing connection. v

Still another feature of the invention resides in the economical use 'ofa multipurpose set of electrical brushes or contacts. I

The above-described and other advantages of the present invention willbe better understood from the following detailed description whenconsidered in connection with the accompanying drawings. I

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings which are to beregarded as merely illustrative:

FIG. 1 is a perspective view of a counter. printer constructedinaccordance with the present invention;

FIG. 2 is a vertical sectionalview of a portion of the counter printertaken on the line 2-2 shown in FIG. I i I FIG. 3 is a sectional viewthrough the portion of the counter printer taken on the line 3-3 shownin FIG.

FIG. 4 is a sectional view of the portion of the counter printer takenon the line 4,4 shown in FIG.

FIG. 5 is a sectional view of the portion of the counter printer takenon the line 5--5 shown in FIG.

FIG. 6 is'an elevational view of a portion of the structure shown inFIG. 2; I I

FIG. 7 is an enlarged elevational view of a decade number w heel shownin FIG. 6; I

FIG. 8 is a vertical sectional view ofthe portion of the counter printertaken on the line, 8-} shown in FIG.

FIG. 9 is an enlarged side elevational view of an electrical brushassembly shown in 2;

FIG. 10 is a vertical'sectional view of the brush assembly taken ontheline 10lII sho wn.in FIG. 9;

FIG. 11 is a greatlyenlargedside elevational view of a pawl shown inFIGS. 2 arld 6;. I I t F IG- 12 isanother elevational view of thepawltaken on the line 1,2--12 shown in FIG. '11

FIG. 13 is a side elevational view of a printed circuit board which isemployed with the apparatus shown in FIG. '2;

FIG. 14 is an end elevational view of the printed circuit board shown inFIG. 13;

FIG. 15 is a rear elevational view of the conductor pattern on theprinted circuit board taken on the line 15-15 shown in FIG. 14;

FIG. 16 is a perspective view of an end portion of a ferromagneticclapper;

FIG. 17 is a perspective view of a portion of a molded plastic arm whichcarries the end of the clapper shown in FIG. 16; and I I FIGS. 18 and 19are side elevational views illustrating two different mountings for anelectrical brush asser n bly.

DESCRIPTION OF THE PREFERRED EMBODIMENT The device of the presentinvention may be employed for any number of severalpurposes or for aplurality of purposes. A ratchet wheel. 10 is shown in FIG. 2. Theposition of ratchet wheel 10 determines the state of the decade 11 shownin FIG. 2. The position of ratchet wheel 10 thus may be employed tostore the state of decade 11 for any purpose. A number wheel 12 and aprint wheel 13 may be omitted in some cases. However, as shown in FIG.2, number wheel 12 has digits thereon which may be the numbers 0 through9, inclusive. Number wheel 12, if desired, may be black having whitenumbers thereon which can be read through a transparent window 14.

Print wheel 13 may have type fonts at 15 with corresponding numbers 0through 9. Additional equipment may be employed with print wheel 13 toproduce a number in ink on a roll of paper. Such additional equipmentmay be identical, if desired, to that shown in U.S. Pat. No. 3,429,258.

As shown in FIG. 1, six decades 11 are provided. However, any numbermore or less than six may be provided, if desired.

Most of' the structure shown in FIG. 2 is mounted on a plate 16. Plate16 may be made of a metal, plastic or other material. Plate 16 is fixedrelative to a front panel 17 by a screw 18 that extends through a hole19 in panel 17 and is threaded into plate 16. Window 14 is fixed topanel 17 by screws 20, shown in FIG. 1. Window 14 covers a rectangularhole 21 through panel 17, as shown in FIG. 2.. Hole 21 is not as wide aspanel 17 i so that screws 20 may be threaded into portions of panel 17.Panel 17 includes a lower portion 22 below screws-20 and an upperportion 23 above screws 20, as shown in FIG. 1. Both of the portions 22and 23 are integral with each other.

As shown in FIG. 2, the decade is operated by a solenoid winding 24which is fixed around a U-shaped ferromagnetic core 25. A printedcircuit board 26, shown in FIG. 13, is fixed to plate 16 by anyconventional fastening means. Mounting rods, not shown, extend throughhole 28. Plate 16 and board 26 thus hold several of the parts shown inFIG. 2 in position.

For example, core 25 is fixed to plate 16 by a screw 25'. A nut isthreaded to screw 25. The nut is not shown. Core 25 is also locatedbetween ribs 29, 30 and 31 cast integrally with plate 16. Projection 32is not shown except in FIGS. 2 and 14.

The solenoid actuator includes a ferromagnetic clapper 33. Clapper 33has an end portion 34 with a surface that is approximately cylindrical,the surface being perpendicular to the plane of the drawing in FIG. 2.The surface of portion 34 is not completely cylindrical. This is truebecause clapper 33 has another portion 35 integral with all portionsthereof that extends upwardly away from portion 34. I

The cylindrical surface of portion 34 has a symmetri cal axis 36.Clapper portion 34 rotates relative to an arm about axis 36. Clapperportion 34 also has a translational movement as well as a rotationalmovement relative to plate 16.

Board 26 can prevent arm 37 from sliding off of pin 38.

Movement of clapper 33 is also restrained by arm 37 in two other ways.In the first place, as shown in FIG. 17, arm 37 has a cylindrical recess39 into which clapper portion 34 is inserted. Further, arm 37 has anoverhang 40, as shown in FIG. 17, which prevents clapper 33 from movingaway from plate 16. Still further, recess 39 extends around clapperportion 34 through an angle B, as shown in FIG; 2, greater than 180.

As shown in FIG. 2, clapper 33 is also limited in movement to the rightby engagement with left end portions 41 and 42 of core 25. Clapper 33 islimited in movement to the left, as viewed in FIG. 2, by an eccentric 43identical to an eccentric 48, also shown in FIG. 2. Eccentrics 43 and 48are both press fit on corresponding respective shafts 44 and 49. Botheccentrics 43 and 48 may have screwdriver slots for adjustment thereof,if desired. a

A metal leaf spring 45 biases arm 37 and urges it to move in acounterclockwise direction around the axis of pine 38, as viewed in FIG.2.

As shown in FIG. 2, 35 is a nonmagnetic adhesive tape fixed to clapper33 from touching end 42 of core 25.

As shown in FIG. 3, plate 16 has a hole 46 therethrough. Eccentric 48has a flange 50 integral therewith which extends over the top of spring45, as viewed in FIG. 3. Eccentric 48 also has an eccentric surface 51which is employed to adjust the deflectionofspring 45 and thereby thebias on arm 37. For this purpose, eccentric 48 has a screwdriver slot at52, as shown in FIG.

As shown in FIGS. 5, arm 37 has a flange 53 which extends over the topof spring 45.

As shown in FIGS. 2 and-4, portion 35 may begin at 54 and terminate at55. Clapper portion 35 may thus be substantially uniform in crosssection along its length. Moreover, this cross section may berectangular as indicated at 56 in FIG. 4. The upper and lower surfacesof portion 35, shown in FIG. 4, may be flat and lower generally circularsurfaces 57 and 58 of clapper 33, shown in FIG. 16, may lie in the sameplane as the upper and lower surfaces 59 and 60 of clapper portion 35,as shown in FIG. 16.

In FIG. 4, note will be taken that because of the angle B, shown in FIG.2, and because of the overhang-40 and plate 16, clapper portion 34 iscompletely trapped in arm recess 39. See FIGS. 2 and 4.

Arm 37 may be made of molded plastic, if desired. Arm 37 has acantilever leaf spring 61 molded integrally therewith which springbiases a pawl 62. Pawl 62 has a cylindrical portion 63 that is rotatableabout an parallel, if desired. Further, if desired, the upper and 8 axis64 fixed relative to arm 37. Arm 37 thus has a cylindrical recess 65similar to recess 39. That is, recess 65 extends around portion 63 ofpawl 62 an angular extent greater than 180 degrees.

.Pawl 62, when arm 37 is oscillated about pin 38, drives ratchet wheel10. Ratchet wheel 10 is moved an integral number of teeth 66, i.e. oneor more, each time winding 24 is energized and deenergized.

The precise point at which ratchet wheel 10 stops after being moved bypawl 62 can be adjusted by a screw 67 threaded through aboss 68 castintegrally with plate 16. Screw 67 thus acts as a stop for pawl 62 whenit is moved downwardly, as viewed in FIG. 2, by the return bias actionof spring 45 on arm 37. Each respective screw 67 in each decade 11,shown in FIG. 1, can thus be adjusted to align all the displayed digitson all of the number wheels of all of the respective decades and/or toalign all the digits of any number which is printed on paper 69 shown inFIG. 1. Such alignments are in the directions indicated by arrows 70 inFIG. 1.

In FIG. 2, a conventional no back is indicated at 71 having an integralspring 72 that engages a boss 73 on plate 16. No back 71 is rotatable ona pin 74 that is fixed relative toplate l6.

Ratchet wheel 10 has gear teeth 75 in addition to ratchet teeth 66.Ratchet wheel 10 also has an integral assembly 76 for supporting a brushassembly. The brush assembly is not shown in FIG. 2 for clarity. Thebrush assembly is mounted in a position somewhat different from thatshown in connection with number wheel 12, but the structure of the brushassembly is identical to that shown in 77 in FIG. 2.

Wheels l0, l2 and 13 may all be made of molded plastic, if desired. Theopposite sides of wheels 12 and 13 may have teeth molded integrallytherewith to engage the teeth 75 of ratchet wheel 10. The teeth ofnumber wheel 12 are indicated at 78 in FIG. 6. The teeth of print wheel13 are indicated at 79 in FIG. 6. The teeth 78 and 79 of the respectivewheels 12 and 13 are not shown in FIG. 2 because they are on the sidesof the wheels 12 and 13 opposite those shown in FIG. 2.

In FIG. 7, number wheel 12 is shown having a portion integral therewithwhich is identified as an assembly 80. Assembly 76 may be identical toassembly 80.

The-view of FIG. 8, as stated previously, is taken on the line 8-8 shownin FIG. 2. However, except for portions 81 and 82 of brush assembly 77,the view of FIG. 8 would be thesame as taken on the line A-A shown inFIG. 7. Thus, number wheel 12 has a cylindrical flange 83 which isintegral with a hub 84 through a disc 85. Hub 84 has'a boss 86 on theright side thereof and assembly on the left side thereof, as viewed inFIG.

The surface indicated at 87 in FIG. 7 is also so indicated in FIG. 8.The same is true of surfaces 89, 90, 91, 92 and 93. In FIG.' 7, notewill be taken that the surfaces 87, 89 and 93 all lie in the same planeand are flat. The surface 88 is also flat and lies in a plane parallelto that of surfaces 87, 89, etc. The surfaces 90 and 92 are flat and liein thesame plane, this plane being parallel to that of surface 88.

As shown in FIG. 8, portions 81 and 82 of brush assembly 77 rest onsurfaces 90 and 92.

' Brush assembly 77 is eccentric. Number wheel 12 is also eccentric. Hub84 has a hole 94 extending completely therethrough. As shown in FIG. 7,surfaces 95 and 97 are flat and lie in a plane perpendicular to theplane of surface 88. Surfaces 96 and 98 are also flat and lie in a planeparallel to that of surfaces 95 and 97. A plane midway between surfaces95 and 96 and parallel to the planes of those surfaces may berepresented by a line 99. A line through the axis 100 of hole 94parallel to line 99 is indicated at 101. The distance C between thelines 99 and 101 represent the eccentricity of the assembly 80. Theassembly 77, shown in FIG. 9, is preferably mounted on assembly 80,shown in FIG. 7. Brushes at 102 can be mounted to be eccentric to theaxis of number wheel 1'2.'Moreover, the position of brush 103 relativeto its mounting on assembly 80 makes it spaced from line 99, shown inFIG. 7, a distance D which is shorter than the spacing of brush 102which is indicated by dimension E;

As shown in FIG. 10, assembly 77'has brushes 102 and 103 fixed to aspring metal copper or other highly conductive blade 104 that hasprojections 105 and 106 that fit against surfaces 95 and 96, shown inFIG. 7. Similarly, blade 104 has projections 107 and 108 which fitcontiguous to surface 97 and 98, respectively, shown in FIG. 7. It is afeature of the present invention that two assemblies identical toassembly 77 may be employed with both number wheel 12 and ratchet'wheel10. However, the eccentricity of number wheel 12 and the eccentricity ofbrush assembly 77 preferably is additive. When a brush assembly ismounted on ratchet wheel 10, the brush assembly ismounted on ratchetwheel with the eccentricity thereof subtracted from the eccentricity ofthe brush assembly. However, preferably, C E D/2. This relationshipdetermines the dimension C whereby brushes 102' and 103 can be mountedon ratchet wheel 10 at locations equidistant from the axis of ratchetwheel 10. See FIGS. 18 and 19. In FIG. 18, (E C) is greater than (D C).Hence, the

brushes are mounted eccentric to the axis. In FIG. 19,

note will be taken that (E C) (D -l- C).

Pawl 62 is again shown in FIGS. 11 and 12. Any conventional pawl may besubstituted for pawl 62. Pawl 62 has a partial cylindrical surface 109to receive spring 61.

As shown in FIG. 14, printed circuit board 26 may be made of aninsulating material 111 and have a pattern of conductors 110 thereon.

In FIG. 15, at one typical position, the brushes carried by number wheel12 may be located at points 112 and 113. At the same time, the brushescarried by ratchet wheel 10 may be located in positions indicated at 114and 115. If desired, brush assemblies for both the number wheel-12 andthe ratchet wheel 10 may operate with conductors 110 shown in F IG. in amanner similar to that disclosed in U.S. Pat. No.3 ,238,359.

In general, the purpose of that portion of the conductor pattern.indicated at 1 16- with the brush assembly for the number wheel 12 is toprovide for transfer'and' The driver of arm 37 is arranged so that whenthe pawl 62 is moved, it is moved one of the teeth 66.

When winding 24 is deenergized after it has been energized, spring 45returns the pawl 62 to the position shown in FIG. 2. Screw 67 acts as anadjustable stop for pawl 62 and ratchet wheel 10 to align all the digitsin a number. It thus permits all the digits in a number'to be aligned.

' Inertia could be a problem. However, this problem is solved by thepresent invention because the spring 45 causes pawl 62 to be wedged inbetween the end of screw 67 and each inclined portion of each tooth ofratchet wheel 10. The ratchet wheel 10 thus cannot overtravel in anadvancing direction because it is immobilized.

Wheels 12 and 13 rotate twice as fast as ratchet wheel 10. i I

Ratchet wheel 10 maybe provided with twenty or twenty-four ratchet teeth66. Plate 16, board 26 and conductors may be nonmagnetic, if desired.

All of the blade assemblies 77 disclosed herein may have-a size andshape to lie-in pressure contact with their corresponding wheels andboard 26 and conductors 110. However, their mountings on theircorresponding wheels may be somewhat loose and rockable;

if desired, to help balance the forces with which the contacts 102 and103 engage board 26 and conductors 110. I What isclaimed is: 1. Anincremental drive mechanism comprising: a

base; an electromagnet fixed relative to said base, said electromagnethaving a ferromagnetic core with a con-.

ductive winding therearound said core having one end a recess therein; aferromagnetic clapperhaving an enlargement on one end thereof trapped insaid recess,.

said base having a portion extending over one end of said recess to trapsaid clapper enlargement, said arm having a portion extending over theother end of said I recess to trap said clapper enlargement, saidclapper having a shank portion connected with said enlarge ment, saidrecess having an opening between the ends thereof larger than said shankportion thereat to permit limited rotation of said clapper, said openingbeing smaller than said enlargement to trap said enlargement in saidrecess, said arm and said clapper being con structed to cause saidenlargement to be rotatable inside said recess only about a second axisapproximately parallel to said first axis, said clapper being located ina position such that said shank portion is drawn toward said core whensaid winding is energized; a toothed ratchet wheel pivoted on said base;means on said arm positioned to rotate said ratchet wheel an integralnuniber of teeth each time said winding is energized and subsequentlydeenergized; and spring means to bias said arm in a direction towardsaid ratchet wheel. I

2. The invention as defined in claim 1, wherein said arm recess is atleast partially defined by an internal surface which is a partialcylindrical surface of revolution, said opening being a radiallyextending slot through said surface of revolution, said slot having auniform angular width less than along the axial length thereof, saidenlargement having a conformal cylindrical surface cut off at its endsby approximately flat and parallel circular surfaces, the internalsurface of the portion of said base covering said one end of said recessbeing approximately flat and parallel to said circular surfaces, theportion of said arm covering said other end of said-recess having anapproximately flat internal surface parallel to said circular surface.

3. The invention as defined in claim 2, including means connected withsaid ratchet wheel to indicate a different symbol each time said windingis energized and deenergized. V Y

4. The invention as defined in claim 3, wherein said clapper is freelymovable and is substantially spaced from all pivotally guidingstructures except for said enlargement being trapped in said recess asaforesaid.

5.' The invention as defined in claim 1, including means connected withsaid ratchet wheel to indicate a different symbol each time said windingis energized and deenergized. I

6. The invention as defined in claim 5, wherein said clapper is freelymovable and is substantially spaced from all pivotally guidingstructures except for said enlargement being trapped in said recess asaforesaid.

7. The invention as defined in claim 1, wherein said clapper is freelymovable and is substantially spaced from all pivotally guidingstructures except for said enlargements being trapped in said recess asaforesaid.

8. An incremental drive mechanism comprising: a base; an electromagnetfixed relative to said base, said electromagnethaving a ferromagneticcore with a conductive winding therearound, said time having one endlocated in a predetermined position; a nonmagnetic about athird axisparallel to said first'and second axes said pawl being positioned toengage successive teeth on said wheel as said clapper and said firstspring oscillate said arm about said first axis when said winding isenergized and deenergized, respectively; a second spring mounted betweensaid arm and said pawl to bias said pawl 'into 'engagement with saidratchet wheel teeth; and anadjustable stop movable on said base to limitratchet wheel advancing movement of said pawl by engagement by saidpawl.

9. The invention as defined in claim 8, including means connected withsaid ratchet wheel to indicate a different symbol each timesaid windingis energized and deenergized. v

10. An incremental drive mechanismcomprising: a base; a ratchet wheelrotatably mounted on said base, said ratchet wheel having teeth; a pawlrotatably mounted relative to said base in a position engageable withsuccessive teeth around said ratchet wheel; means to oscillate said pawlfor engagement with successive ratchet wheel teeth to advance the samestep by step;

a bracket fixed relative to said base; and a stop having a screwthreaded through said bracket for engagement by and to limit advancingmovement of said pawl.

11. The invention as defined in claim 10, wherein said base includes aportion having a hole threaded through, said stop including a screwthreaded through said hole. 7

12. An incremental drive mechanism comprising: a base; an electromagnetfixed relative to said base, said electromagnet having a ferromagneticcore with a conductive winding therearound said core having one endlocated in a predetermined position; a nonmagnetic arm; means mountingsaid'arm on said base in a position pivotable about a first axis fixedrelative to said base but otherwise in a fixed position relative to saidbase in a direction along said first axis, said arm having a recesstherein; a ferromagnetic clapper having an enlargement on one endthereof in said recess, said clapper having a shank portion connectedwith said enlargement, said recess having an opening between the endsthereof larger than said shank portion thereat to permit limitedrotation of said clapper, said arm and said clapper being constructed tocause said enlargement to be rotatable inside said recess only about asecond axis approximately parallel to said first axis, said clapperbeing located in a position such that said shank portion is drawn towardsaid core when said winding is energized; a toothed ratchet wheelpivoted on said base; means on said arm positioned to rotate saidratchet wheel an integral number of teeth each time said winding isenergized and subsequently deenergized; and spring means to bias saidarm in a direction toward said ratchet wheel.

13. The invention as defined in claim 12, wherein said arm recess is atleast partially defined by an internal surface which is a partialcylindrical surface of revolution, said opening being a radiallyextending slot through said surface of revolution, said slot having anangular, width less than degrees along the axial length thereof andhaving a confonnal cylindrical surface.

14. The invention as defined in claim 13, including means connected withsaid ratchet wheel to indicate a difi'erent symbol each time saidwinding is energized and deenergized.

15. The invention as defined in claim 14, wherein said clapper ispivotally guided but only on said arm.

16. The invention as defined in claim 15, wherein said opening issmaller than said enlargement to trap said'enlargement in said recess.

'17. The invention as defined in claim 12, wherein said opening issmaller than said enlargement to trap said enlargement in said recess.

18. A decade comprising: a base; a first wheel rotatably mounted "onsaid base, said first wheel having a first assembly thereon eccentric tothe first wheel axis; a first wiper blade having a first pair of brushesfixed thereto in positions spaced apart along a first line therebetween,said first wiper blade having first mounting means closer to one brushthan the other said first mounting meansbeing supported on said firstassembly in a manner such that said first line passes through the axisof said first wheel; and a printed circuit board fixed relative to saidbase and having conductors positioned to be engaged by said brushes.

19. The invention as defined in claim 18, including a second wheelrotatably mounted on said base and having a second assemblysubstantially identical to said first assembly, a second wiper bladehaving a second pair of brushes and being substantially identical tosaid first wiper blade having a second mounting means sub stantiallyidentical in size, shape and location between said second wiper bladebrushes to the size, shape and location of said first mounting meansbetween said first wiper blade brushes, said second mounting means beingsupported on said second assembly in a manner such that a second linebetween said second wiper blade brushes passes through the axis of saidsecond wheel.

20. The invention as defined in claim 19, wherein said one brush of eachof said pairs is located a distance D from each respective mountingmeans, the other brush of each of said pairs being located a distance Efrom each respective mounting means, the eccentricity of each saidassembly being C, said one brush of said first pair being spaced adistance from said first wheel axis less than the distance said onebrush of said second pair is spaced from said second wheel axis.

21. The invention as defined in claim 20, wherein C is defined by theequation 22. A decade comprising: a base; a ratchet wheel rotatable onsaid base; a pawl movably mounted on said base, said wheel having teethwith surfaces inclined relative to a diameter; a stop mounted on saidbase adjacent said teeth; and power means to advance said wheel bymoving said pawl, said pawl being moved in a manner such that it isultimately wedged between said stop and one of said inclined surfaces toprevent overtravel of said wheel in an advancing direction.

23. The invention as defined in claim 22, wherein said power meansincludes a spring to bias said pawl toward the position in which it iswedged as aforesaid.

1. An incremental drive mechanism comprising: a base; an electromagnetfixed relative to said base, said electromagnet having a ferromagneticcore with a conductive winding therearound said core having one endlocated in a predetermined position; a nonmagnetic arm; means mountingsaid arm on said base in a position pivotable about a first axis fixedrelative to said base but otherwise in a fixed position relative to saidbase in a direction along said first axis, said arm having a recesstherein; a ferromagnetic clapper having an enlargement on one endthereof trapped in said recess, said base having a portion extendingover one end of said recess to trap said clapper enlargement, said armhaving a portion extending over the other end of said recess to trapsaid clapper enlargement, said clapper having a shank portion connectedwith said enlargement, said recess having an opening between the endsthereof larger than said shank portion thereat to permit limitedrotation of said clapper, said opening being smaller than saidenlargement to trap said enlargement in said recess, said arm and saidclapper being constructed to cause said enlargement to be rotatableinside said recess only about a second axis approximately parallel tosaid first axis, said clapper being located in a position such that saidshank portion is drawn toward said core when said winding is energized;a toothed ratchet wheel pivoted on said base; means on said armpositioned to rotate said ratchet wheel an integral number of teeth eachtime said winding is energized and subsequently deenergized; and springmeans to bias sAid arm in a direction toward said ratchet wheel.
 2. Theinvention as defined in claim 1, wherein said arm recess is at leastpartially defined by an internal surface which is a partial cylindricalsurface of revolution, said opening being a radially extending slotthrough said surface of revolution, said slot having a uniform angularwidth less than 180* along the axial length thereof, said enlargementhaving a conformal cylindrical surface cut off at its ends byapproximately flat and parallel circular surfaces, the internal surfaceof the portion of said base covering said one end of said recess beingapproximately flat and parallel to said circular surfaces, the portionof said arm covering said other end of said recess having anapproximately flat internal surface parallel to said circular surface.3. The invention as defined in claim 2, including means connected withsaid ratchet wheel to indicate a different symbol each time said windingis energized and deenergized.
 4. The invention as defined in claim 3,wherein said clapper is freely movable and is substantially spaced fromall pivotally guiding structures except for said enlargement beingtrapped in said recess as aforesaid.
 5. The invention as defined inclaim 1, including means connected with said ratchet wheel to indicate adifferent symbol each time said winding is energized and deenergized. 6.The invention as defined in claim 5, wherein said clapper is freelymovable and is substantially spaced from all pivotally guidingstructures except for said enlargement being trapped in said recess asaforesaid.
 7. The invention as defined in claim 1, wherein said clapperis freely movable and is substantially spaced from all pivotally guidingstructures except for said enlargements being trapped in said recess asaforesaid.
 8. An incremental drive mechanism comprising: a base; anelectromagnet fixed relative to said base, said electromagnet having aferromagnetic core with a conductive winding therearound, said corehaving one end located in a predetermined position; a nonmagnetic arm;means mounting said arm on said base in a position pivotable about afirst axis fixed relative to said base but otherwise in a fixed positionrelative to said base, said arm being pivoted around said first axis ina predetermined direction when said winding is energized; a first springmounted between said base and said arm to bias said arm in a directionopposite to said predetermined direction; a ratchet wheel pivoted onsaid base about a second axis parallel to said first axis, said ratchetwheel having teeth; a pawl pivoted on said arm about a third axisparallel to said first and second axes, said pawl being positioned toengage successive teeth on said wheel as said clapper and said firstspring oscillate said arm about said first axis when said winding isenergized and deenergized, respectively; a second spring mounted betweensaid arm and said pawl to bias said pawl into engagement with saidratchet wheel teeth; and an adjustable stop movable on said base tolimit ratchet wheel advancing movement of said pawl by engagement bysaid pawl.
 9. The invention as defined in claim 8, including meansconnected with said ratchet wheel to indicate a different symbol eachtime said winding is energized and deenergized.
 10. An incremental drivemechanism comprising: a base; a ratchet wheel rotatably mounted on saidbase, said ratchet wheel having teeth; a pawl rotatably mounted relativeto said base in a position engageable with successive teeth around saidratchet wheel; means to oscillate said pawl for engagement withsuccessive ratchet wheel teeth to advance the same step by step; abracket fixed relative to said base; and a stop having a screw threadedthrough said bracket for engagement by and to limit advancing movementof said pawl.
 11. The invention as defined in claim 10, wherein saidbase includes a portion having a hole threaded through, said stopincluding a screw threAded through said hole.
 12. An incremental drivemechanism comprising: a base; an electromagnet fixed relative to saidbase, said electromagnet having a ferromagnetic core with a conductivewinding therearound said core having one end located in a predeterminedposition; a nonmagnetic arm; means mounting said arm on said base in aposition pivotable about a first axis fixed relative to said base butotherwise in a fixed position relative to said base in a direction alongsaid first axis, said arm having a recess therein; a ferromagneticclapper having an enlargement on one end thereof in said recess, saidclapper having a shank portion connected with said enlargement, saidrecess having an opening between the ends thereof larger than said shankportion thereat to permit limited rotation of said clapper, said arm andsaid clapper being constructed to cause said enlargement to be rotatableinside said recess only about a second axis approximately parallel tosaid first axis, said clapper being located in a position such that saidshank portion is drawn toward said core when said winding is energized;a toothed ratchet wheel pivoted on said base; means on said armpositioned to rotate said ratchet wheel an integral number of teeth eachtime said winding is energized and subsequently deenergized; and springmeans to bias said arm in a direction toward said ratchet wheel.
 13. Theinvention as defined in claim 12, wherein said arm recess is at leastpartially defined by an internal surface which is a partial cylindricalsurface of revolution, said opening being a radially extending slotthrough said surface of revolution, said slot having an angular widthless than 180 degrees along the axial length thereof and having aconformal cylindrical surface.
 14. The invention as defined in claim 13,including means connected with said ratchet wheel to indicate adifferent symbol each time said winding is energized and deenergized.15. The invention as defined in claim 14, wherein said clapper ispivotally guided but only on said arm.
 16. The invention as defined inclaim 15, wherein said opening is smaller than said enlargement to trapsaid enlargement in said recess.
 17. The invention as defined in claim12, wherein said opening is smaller than said enlargement to trap saidenlargement in said recess.
 18. A decade comprising: a base; a firstwheel rotatably mounted on said base, said first wheel having a firstassembly thereon eccentric to the first wheel axis; a first wiper bladehaving a first pair of brushes fixed thereto in positions spaced apartalong a first line therebetween, said first wiper blade having firstmounting means closer to one brush than the other said first mountingmeans being supported on said first assembly in a manner such that saidfirst line passes through the axis of said first wheel; and a printedcircuit board fixed relative to said base and having conductorspositioned to be engaged by said brushes.
 19. The invention as definedin claim 18, including a second wheel rotatably mounted on said base andhaving a second assembly substantially identical to said first assembly,a second wiper blade having a second pair of brushes and beingsubstantially identical to said first wiper blade having a secondmounting means substantially identical in size, shape and locationbetween said second wiper blade brushes to the size, shape and locationof said first mounting means between said first wiper blade brushes,said second mounting means being supported on said second assembly in amanner such that a second line between said second wiper blade brushespasses through the axis of said second wheel.
 20. The invention asdefined in claim 19, wherein said one brush of each of said pairs islocated a distance D from each respective mounting means, the otherbrush of each of said pairs being located a distance E from eachrespective mounting means, the eccentricity of each said assembly beingC, said one brush of said first pair being spaced a distAnce from saidfirst wheel axis less than the distance said one brush of said secondpair is spaced from said second wheel axis.
 21. The invention as definedin claim 20, wherein C is defined by the equation C E - D/2.
 22. Adecade comprising: a base; a ratchet wheel rotatable on said base; apawl movably mounted on said base, said wheel having teeth with surfacesinclined relative to a diameter; a stop mounted on said base adjacentsaid teeth; and power means to advance said wheel by moving said pawl,said pawl being moved in a manner such that it is ultimately wedgedbetween said stop and one of said inclined surfaces to preventovertravel of said wheel in an advancing direction.
 23. The invention asdefined in claim 22, wherein said power means includes a spring to biassaid pawl toward the position in which it is wedged as aforesaid.